This invention relates to an apparatus and methods for separating a fluid from a mixture of fluids, in particular, down-hole separation of multi-phase fluids produced by an oil well.
Many oil wells produce what is basically a mixture of fluids. It is notably a mixture of oil and/or gas together with water, said mixture sometimes carrying mineral particles, especially sand. Somewhere in the oil production system, the fluids of said mixture have to be separated. Then, the oil and/or the gas is or are stored for deliverance to a refinery for further treatment and the water is disposed of, after treatment, at the surface or pumped back into the oil well or another oil well adjacent to the one which it was removed from.
The fluid separation may take place at the surface facilities, the sea bed or down-hole. Separation at the surface is cost effective, especially as water production tends to increase as wells become depleted of oil. In addition, it diminishes the oil and/or gas potential volume to be lifted at the surface per time unit. These are the reasons why down-hole separation is generally preferred, even if remoteness of the separation process from control at the surface, difficulty to carry out down-hole maintenance and the restricted geometry in the down-hole environment constitute sensitive issues that are to be addressed for the implementation of down-hole separation methods.
Several known methods have been proposed for down-hole fluid separation. Amongst these, a number allow fluid separation under the effect of the gravity.
For highly deviated wells, where the deviation is greater than 70xc2x0 from vertical, the fluid mixture flowing along the pipe is partially stratified and each fluid tends to have a fluid concentration gradient from top to bottom. The known methods using gravity therefore generally propose to achieve separation using carefully designed outlet pipes. For example, in the document published on Aug. 18, 1999 under the number GB 2 326 895, it is proposed to draw the fluids from the flowing mixture using carefully designed outlet pipes. Measurements provided by detectors permit the adjustment of the flow rate through the pipes. In other examples, more complicated manifolds are used. They guarantee the removal of water from the lowest point in the well bore. However, when the deviation of the wells is of about 85 to 90xc2x0 from vertical, it may be advantageous, as shown in the document published on Sep. 24, 1998 under the number WO 98/41304, to drill oversize a large area of the well bore wherein the residence time of the mixture is increased, leading to an enhanced water separation rate under the effect of gravity.
For lower deviated wells, where the deviation is less than 70xc2x0, refined methods further involve placing flat or corrugated parallel plates, themselves parallel to the mixture flow. Due to the presence of said plates, the Reynolds number of the mixture is reduced and new surfaces for coalescence are provided, thus accelerating the separation of water.
The above methods are finally particularly adapted to a specific well geometry, having a specific deviation, and to specific mixture flow rate and flow regime.
Considering the prior art, one problem that the invention is proposing to solve is to carry out a down-hole apparatus for separation of a fluid from a mixture of fluid, as well as a method for doing the same, said apparatus and method providing fluid separation for all well deviations, whatever be the mixture flow rate and flow regime into the producing pipe.
In a first aspect, the invention concerns an apparatus for separation of a first fluid from a mixture of at least said first fluid and a second fluid, comprising:xe2x80x94a production pipe for flowing the mixture;xe2x80x94a plurality of settling chambers surrounding said production pipe;xe2x80x94for each settling chamber at least one aperture allowing at least part of the mixture flowing in the production pipe to flow into the settling chamber; andxe2x80x94an exit pipe, connected to one or more of the settling chambers, allowing the first fluid to flow out of said settling chambers. The apparatus is preferably designed and arranged to be installed and operated down-hole.
In a second aspect, the invention concerns a method for separation of a first fluid from a mixture of at least said first fluid and a second fluid, comprising the following steps: flowing the mixture through a production pipe; flowing at least part of the mixture through at least one aperture of said production pipe, into a plurality of settling chambers surrounding said production pipe; allowing the first fluid to separate from the mixture under the effect of the gravity in each of said settling chambers; and flowing the first fluid out of the settling chambers. The method is preferably a down-hole separation method and the aforementioned steps are preferably performed down-hole.
The settling chambers are to some extent independent from the producing pipe. Therefore, the flow rate and regime of the mixture circulating into said pipe are also independent from the mixture flow rate and regime into the chamber. In particular, whatever is the flow rate into the producing pipe, the fluid mixture contained in the chamber may almost stagnate so as to allow gravity to separate the fluids.
Also, the separation may take place independently of the pipe deviation.
According to another embodiment, a method and apparatus is provided for separation on the surface. As used herein, the term xe2x80x9csurfacexe2x80x9d in marine applications includes at and above the sea-bed. The structures and features of the described down-hole separator can, in many cases, also be applied to surface separation applications.